Low energy electron ranges in liquids: determination by nonhomogeneous kinetics

Abstract

Free ion yields have been measured in four hydrocarbon liquids: n-pentane, cyclopentane, neopentane, and neohexane. Each liquid has been studied from room temperature or below up to the critical temperature. Theoretical curves have been calculated using the relation between the free ion yields and the external field strength derived by Terlecki and Fiutak on the basis of an equation by Onsager. Two popular electron range distribution functions, Gaussian and exponential, have been shown not to be an adequate representation of the reality as far as the model used for the calculations is concerned. In order to fit experimental points, both range distribution functions would require a drastic increase in the total ionization yield, Gtot, with temperature increase. This would mean an unrealistic decrease of the ionization potential of the molecule from the melting point up to the critical temperature.It is possible to keep Gtot quite constant and within the range of values obtained by other techniques by extending the Gaussian range distribution function with a (range)−3 probability tail. The most probable range can be normalized for the liquid density. This parameter has been used to obtain information about the behaviour of epithermal electrons in the four alkane liquids from the melting point up to the critical temperature.(1) Normalized penetration ranges of epithermal electrons are dependent on the structure of the molecule in the entire liquid range but differences are smaller at critical than at low temperatures.(2) Normalized penetration ranges of epithermal electrons pass through a maximum in the liquid phase for neopentane, neohexane, and cyclopentane. No maximum is observed for n-pentane.(3) There is no drastic change in the behaviour of epithermal electrons in these alkanes at the critical temperature.